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  • 1980-1984  (16)
  • 1
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 303 (1983), S. 756-756 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] THE tracking of Earth satellites continues to produce new findings about Earth pro perties. The most recent finding, reported in this issue of Nature (p.757), uses 5.5 years' laser ranging to Lageos, a 60-centimetre ball of brass covered by retro-reflectors whose retrograde orbit lies at 6,000 km ...
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2019-01-25
    Description: In models of lunar origin by great impact, attention is usually paid to the hydrodynamic expansion resulting from the great amount of thermal energy. However, the source of this disruption is inevitably significant in a close approach between large bodies, it is to be expected that significant departures from simple hyperbolic orbits would occur even before impact. These departures could arise from mechanical effects, and hence purely mechanical models are worth pursuing. The most interesting results obtained for approach offsets are small multiples of the planet radius and approach velocities of a few kilometers/second. In an interaction between Mars and Earth sized protoplanets, most of the material ends in collision, but a few percent end in elliptic orbits and a few percent escape. Another model considered is an offset collision, arising from a wide range of approach velocities and offsets.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar Planetary Inst. Conf. on the Origin of the Moon; p 59
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  • 3
    Publication Date: 2019-01-25
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: HIG-CONTRIB-1244-VOL-2 , Hawaii Univ. Contrib. of the Hawaii Inst. of Geophys., Vol. 2; 27 p
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  • 4
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    Publication Date: 2011-08-18
    Description: The long-wave features of the gravity field of Venus differ from those of the earth's field not only in their strong positive correlation with topography, but also in their gentler spectral slope. These properties are inconsistent with generation of the gravity field by plate tectonics or by processes at great depths; they are consistent with generation by a mantle convective system supporting the broad features in topography with an effective compensation depth of about 450 km.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 11; 35-37
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  • 5
    Publication Date: 2011-08-18
    Description: Heat-flow and plate-velocity measurements are used to model upper-mantle temperature variations; the results are intended to form the basis for a gravity-field-variation model and are also applicable to the interpretation of petrological and seismological data. A 5-deg grid is used, a depth of 280 km is chosen as the fully convecting level, and it is assumed that 85 percent of the global heat production (4.0 x 10 to the 13th W) comes from below this level. The velocity field is calculated by integrating the momentum equations down to 280 km (assuming laterally homogeneous viscosity and density) and then used to determine the temperature fields. The results are presented in graphs, tables, and maps. The largest lateral temperature differences are about 1500 C and occur in the top 20 km, where the largest (about 200 C/Myr) nonlinear terms of the energy equation are also found. Below 50 km, heat transfer becomes more convective than conductive, and the most significant temperature variation appears in the form of negative 'tongues' as cold as -825 C at about 100 km. Temperature variations of at least + or - 180 C are calculated at the fully convecting level.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 88; 10323-10
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  • 6
    Publication Date: 2011-08-18
    Description: An analytic scheme for inferring variations of the gravity field from satellite-to-satellite range rate (low-low) is developed. As a test, it is applied to a pair of satellites in polar orbit, at altitude 160 km and spacing 100 km, with 72 data points per revolution. An assumed gravity field of tesseral spherical harmonics up to the eighth degree is completely recovered in three iterations over 64 revolutions. It is apparent that data points at regular intervals enable the use of data analysis techniques that avoid massive matrix inversions.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 88; Oct. 10
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  • 7
    Publication Date: 2011-08-18
    Description: The present investigation is supplementary to a study reported by Kaula and Phillips (1981). From an analysis of Pioneer Venus altimetry, Kaula and Phillips had inferred that any heat loss from the planet by plate tectonics must be small compared to that from the earth. However, it has been suggested by others that plate tectonic may exist on Venus, but that the expected 'square root of s' dependence of the topographic drop off is not observed because it is concealed by lava flows. The present investigation has the objective to conduct an examination whether this suggestion of concealment by lava flow is correct. On the basis of the performed analysis, it is concluded that the results obtained by Kaula and Phillips appear to be well justified.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 9; Sept
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  • 8
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    Publication Date: 2011-08-18
    Description: An analytical theory is developed for the velocity evolution of nonaccreting planetesimal populations, based on the Boltzmann and Fokker-Planck equations. Adapting Shkarofsky's calculation of plasma viscosities, the rate of increase in random velocities due to gravitational encounters between planetesimals of equal mass is found to be one-third to one-half Safronov's result. Comparison with Wetherill's numerical experiments suggests that the Fokker-Planck equation underestimates the effectiveness of encounters and that Safronov's value is approximately correct. For populations of nonuniform sizes, the Fokker-Planck equation indicates an efficient redistribution of energy from the largest bodies to the smaller ones. By conserving angular momentum, the rate of radial spreading of orbits is also derived.
    Keywords: ASTRONOMY
    Type: Icarus; 44; Oct. 198
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  • 9
    Publication Date: 2011-08-18
    Description: Properties of the earth are considered in the light of the earth's presumed status as an end member of the class of predominantly silicate solar system bodies. Bulk composition data for the inner planets are presented and processes which may account for compositional variations are discussed. Problems posed by Viking findings on Mars and Pioneer findings on Venus are mentioned. Prominent evolutionary patterns of the inner planets are discussed, including the inverse correlation of lithospheric and crustal thickness with planetary size, and the thermotectonics of Venus, Mars, and earth are compared.
    Keywords: LUNAR AND PLANETARY EXPLORATION
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  • 10
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    Publication Date: 2011-08-18
    Description: Quantitative comparisons are made between the characteristics of plate tectonics on the earth and those which are possible on Venus. Considerations of the factors influencing rise height and relating the decrease in rise height to plate velocity indicate that the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock-fluid density contrast and lower temperature differential between the surface and interior. Statistical analyses of Pioneer Venus radar altimetry data and global earth elevation data is used to identify 21,000 km of ridge on Venus and 33,000 km on earth, and reveal Venus ridges to have a less well-defined mode in crest heights and a greater concavity than earth ridges. Comparison of the Venus results with the spreading rates and associated heat flow on earth reveals plate creation rates on Venus to be 0.7 sq km/year or less and indicates that not more than 15% of Venus's energy is delivered to the surface by plate tectonics, in contrast to values of 2.9 sq km a year and 70% for earth.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters; 8; Dec. 198
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